The present invention generally relates to wire-bonded substrate assemblies and methods of producing the same, and more particularly to a wire-bonded substrate assembly which is wire-bonded on both sides of the substrate and a method of producing such a wire-bonded substrate assembly.
In order to reduce the size of the substrate assembly, the chip-on-board method is employed to mount an integrated circuit (IC) chip directly on the substrate. But there are now demands to reduce the size of the substrate assembly so as to enable further improvement in the integration density.
FIG. 1 shows an example of a conventional wire-bonded substrate assembly. A wire-bonded substrate assembly 1 shown in FIG. 1 is made up of a substrate 2 having a surface 3. IC chips 4 and 5 and a capacitor 6 are mounted on the surface 3. The IC chips 4 and 5 respectively have wires 7 and 8 bonded thereon. The wire-bonded IC chips 4 and 5 are respectively encapsulated by synthetic resins 9 and 10.
The wires 7 and 8 are bonded by placing the substrate 2 on a flat work table 12 which has a built-in heater 11, and using an ultrasonic bonding tool (not shown) while heating the substrate 2 from its bottom by the heater 11.
According to the wire-bonded substrate assembly 1, the IC chips 4 and 5 are mounted on only the surface 3 of the substrate 2. For this reason, there is a limit to improving the mounting density and it is difficult to further reduce the size of the wire-bonded substrate assembly 1.
In order to solve the problem of the wire-bonded substrate assembly 1 described above, it is conceivable to mount IC chips on the other surface 13 of the substrate 2. However, if the substrate 2 is placed on the word table 12 with the surface 3 facing down as shown in FIG. 2, the substrate 2 is in effect supported by the synthetic resins 9 and 10 and a gap 14 is inevitably formed between the substrate 2 and the top of the work table 12. In FIG. 2, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
As a result, a bonding part of the substrate 2 on the surface 13 where the wire-bonding is to be made cannot be heated satisfactorily by the heater 11 due to the existence of the gap 14, and the ultrasonic waves applied to this bonding part by the ultrasonic bonding tool cannot be concentrated at the bonding part. Instead, the ultrasonic waves tend to propagate to the parts surrounding the bonding part. In addition, the substrate 2 on the work table 12 is supported by the synthetic resins 9 and 10, and the support of the substrate 2 is not sufficiently stable. For the above described reasons, there is a problem in that the ultrasonic wire-bonding cannot be made stably. Accordingly, it was conventionally impossible to mount IC chips on both the surfaces 3 and 13 of the substrate 2 by the wire-bonding.